Liquid Crystal Display

A liquid crystal display (LCD) is a type of display which uses the light modulating (polarizing) properties of liquid crystals. They are semi-solid crystals arranged on a substrate. Liquid crystals do not emit light directly, they are the medium that helps the light to bend/rotate. An LCD screen consist of a large number of small pixels, each having a combination of red, green and blue sub-pixels. LCD uses the property of persistance of vision of Human eye due to which we see the video.

The LCD module is made up of a mesh of liquid crystals and arrayed in front of a light source (usually at the bottom of the screen) The reflector is used to distribute the light evenly through the screen area. A liquid crystal cell consists of a thin layer (about 10 u m) of a liquid crystal sand­wiched between two glass sheets with transparent electrodes deposited on their inside faces. With both glass sheets transparent, the cell is known as transmitting type cell. When one glass is transparent and the other has a reflective coating, the cell is called reflective type. The LCD does not produce any illumination of its own.

Working of lcd

The main principle behind liquid crystal molecules is that when an electric current is applied to them, they tend to untwist. This causes a change in the light angle passing through them. This causes a change in the angle of the top polarizing filter with respect to it. So little light is allowed to pass through that particular area of LCD. Thus that area becomes darker comparing to others.
For making an LCD screen, a reflective mirror has to be setup in the back. An electrode plane made of indium-tin oxide is kept on top and a glass with a polarizing film is also added on the bottom side.

The entire area of the LCD has to be covered by a common electrode and above it should be the liquid crystal substance. Next comes another piece of glass with an electrode in the shape of the rectangle on the bottom and, on top, another polarizing film. It must be noted that both of them are kept at right angles. When there is no current, the light passes through the front of the LCD it will be reflected by the mirror and bounced back. As the electrode is connected to a temporary battery the current from it will cause the liquid crystals between the common-plane electrode and the electrode shaped like a rectangle to untwist. Thus the light is blocked from passing through. Thus that particular rectangular area appears blank.

Advantages:

Low power consumption: Power consumption is typically of the order of micro watts for the display in comparison to the some order of milliwatts for LEDs.

Sharpness: Image is perfectly sharp at the native resolution of the panel. LCDs using an analog input require careful adjustment of pixel tracking/phase (see Interference, below).

Geometric Distortion: Zero geometric distortion at the native resolution of the panel. Minor distortion for other resolutions because the images must be re-scaled.

Brightness: High peak intensity produces very bright images. Best for brightly lit environments.

Physical
Thin, with a small footprint. Consume little electricity and produce little heat.

Disadvantages

Resolution: Each panel has a fixed pixel resolution format determined at the time of manufacture that can not be changed.

Interference: LCDs using an analog input require careful adjustment of pixel tracking/phase in order to reduce or eliminate digital noise in the image.

Viewing Angle: Limited viewing angle. Brightness, contrast, gamma and color mixtures vary with the viewing angle. Can lead to contrast and color reversal at large angles. Need to be viewed as close to straight ahead as possible.

Black-Level, Contrast and Color Saturation: LCDs have difficulty producing black and very dark grays. As a result they generally have lower contrast than CRTs and the color saturation for low intensity colors is also reduced. Not suitable for use in dimly lit and dark environments.

White Saturation: The bright-end of the LCD intensity scale is easily overloaded, which leads to saturation and compression. When this happens the maximum brightness occurs before reaching the peak of the gray-scale or the brightness increases slowly near the maximum. Requires careful adjustment of the Contrast control.

Aspect Ratio: LCDs have a fixed resolution and aspect ratio. For panels with a resolution of 1280x1024 the aspect ratio is 5:4=1.25, which is noticeably smaller than the 4:3=1.33 aspect ratio for almost all other standard display modes. For some applications may require switching to a letterboxed 1280x960, which has a 4:3 aspect ratio.

Cost: Considerably more expensive than comparable CRTs.

Applications:

Clocks: Used in displays in digital wall clocks

Cameras: Used in digital cameras for displaying images

Television: Has a major application in LCD TFT screens

Computer monitors: Most of the modern world computer monitors are made of LCDs

LCD interfacing with Micro controller

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